Plasma power supply for arc discharge device

ABSTRACT

A plasma power supply for use in a starting and operating system for a high-intensity, high-pressure arc discharge lamp, such as a xenon lamp. A high-voltage, alternating current source is serially coupled in the lamp circuit for starting the lamp, and a low-voltage, direct current source is coupled across the lamp circuit by a current-limiting ballast resistor and isolating diode. The plasma power supply comprises a plurality of plasma voltage supply circuits respectively having one side coupled to progressively higher voltage points on a transformer secondary winding by diodes, the other sides being common and coupled to a common point on the transformer winding. Each of the supply circuits has a current-limiting resistor therein, the common side of the supply circuits being connected to one side of the lowvoltage source and the other sides being connected together and to the lamp circuit. The supply circuit resistors are proportioned to back-bias the diodes so as sequentially to apply progressively lower voltages to the lamp circuit in response to increase in the current flow in the lamp following starting, this proportioning being such that the voltage/current characteristic of the applied voltages generally approximates the voltage/current characteristic of the lamp.

United States Patent 3,476,977 11/1969 Hallay Inventors Appl. No.

Filed Patented Assignee PLASMA POWER SUPPLY FOR ARC DISCHARGE Stephen J. Erst;

Ralph H. Beardsley, both oi Fort Wayne, Ind.

May 12; 1969 Oct. 5, 1971 International Telephone and Telegraph Corporation Nutley, NJ

I Pri rndry Examiner-Roy Lake ABSTRACT: A plasma power supply for use in a starting and operating system for a high-intensity, high-pressure arc discharge lamp, such as a xenon lamp. A high-voltage, alternating current source is serially coupled in the lamp circuit for starting the lamp, and a low-voltage, direct current source is coupled across the lamp circuit by a current-limiting ballast resistor and isolating diode. The plasma power supply comprises a plurality of plasma voltage supply circuits respectively having one side coupled to progressively higher voltage points on a transformer secondary winding by diodes, the other sides being common and coupled to a common point on the transformer winding. Each of the supply circuits has a currentlimiting resistor therein, the common side of the supply circuits being connected to one side of the low-voltage source and the other sides being connected together and to the lamp circuit. The supply circuit resistors are proportioned to backbias the diodes so as sequentially to apply progressively lower voltages to the lamp circuit in response to increase in the current flow in the lamp following starting, this proportioning being such that the voltage/current characteristic of the applied voltages generally approximates the voltage/current characteristic of the lamp.

g I4 I l 0 2 I2 INVERTER PATENTED our 5 l97| SHEET 2 or 2 RALPH Q/ m l w 22 EEm z o g @N Nw ow m llllllllllllllllllllllllllllllllllllllll allJ m? MN w BTW W m m wm s) wwfl rim mm wm ATTORNEYS PLASMA POWER SUPPLY FOR DISCHARGE DEVICE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to starting and operating systems for are discharge devices, and more particularly to a plasma power supply for use in such systems.

2. Description of the Prior Art High-pressure, high-intensity arc lamps,'such as xenon, mercury, or mercury-xenon arc lamps, require a very high voltage in the general range from to 60 kv. for initial ionization of the gas, i.e. to start" the lamp. Immediately upon application of the high-voltage starting pulse, interclectrode, wirelike discharges appear causing local hotspot heating of the cathode which thus becomes an emitter of electrons with the potential drop across the electrodes becoming lower. As soon as the arc is thus fonned, a much lower voltage in the general range from 65 to 140 volts will sustain and support buildup of the plasma" arc. Finally, when the arc current has built 'up to its operating value, a still lower voltage in the general range from 20 to 35 volts will sustain the are at its operating current level.

Thus, the starting and operating system for such lamps must supply three distinct requirements: the high starting voltage; the much lower plasma supply voltage; and the still lower operating voltage. While the starting voltage for such lamps may be an alternating current voltage, the plasma and operating voltages must be direct current since the arc cannot be sustained from an alternating current source.

Such lamps, in common with all are discharge devices, have a negative voltage/current exponential characteristic, i.e. the interelectrode resistance decreases exponentially as the arc current increases, and thus it is necessary to provide a currentlimiting impedance or ballast in the lamp circuit in order to provide the desired operating current and to prevent ultimate destruction of the lamp. Prior starting and operating systems for such lamps known to the present applicants have employed a plasma power supply providing a single direct current voltage in the general range from I20 to 140 volts. We have observed that there is a tendency for the plasma arc to blow out of the direct path between the lamp electrodes thereby reducing the probability of completing the ignition. Further, with such a relatively high level of voltage, it is difiicult to provide sufficient energy to the interelectrode space to cause proper cathode heating for ignition without wasting an unduly high amount of energy in the ballasting impedance. The ballast impedance is generally a series resistor which provides a linear voltage/current characteristic, a high-reactance transformer, or some combination thereof. None of these conventional ballasting techniques provides a voltage/current characteristic corresponding to that of the lamp and thus, excessive energy is consumed in the ballast in order to provide the requisite limitation of lamp current.

SUMMARY OF THE INVENTION In accordance with the invention, we provide a plasma power supply having a plurality of output circuits respectively providing progressively higher direct current voltages. These supply circuits are sequentially coupled to the lamp so as to apply progressively lower voltages thereto in response to increase in the plasma current flow. Further, a ballast resistor is provided in each of the plasma voltage supply circuits, these resistors being proportioned so that the composite voltage/current characteristic generally approximates that of the lamp.

It is accordingly an object of the present invention to provide an improved plasma power supply for use in a starting and operating system for an arc discharge device.

It is a further object of the invention to provide an improved plasma power supply having a voltage/current characteristic generally approximating that of the arc discharge device.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is aschematic block diagram showing a starting and operating system for an arc discharge device incorporating the plasma power supply of the invention;

FIGS. 2A, B and C show the voltage/current characteristics provided by conventional ballasting techniques;

FIG. 3 shows the voltage/current characteristic of a typical high-pressure, high-intensity arc discharge lamp;

FIG. 4 is a schematic diagram showing one embodiment of the plasma power supply of the invention;

FIG. 5 shows the voltage/current characteristic provided by the plasma power supply of FIG. 4 and FIG. 6 is a schematic diagram showing a modification of the system of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a starting and operating system 10 having its output terminals 11 and 12 connected across the anode l3 and cathode 14 of a high-pressure, high-intensity arc discharge lamp 15, such as a xenon lamp. A high-voltage ignition circuit 16 is coupled in series with anode l3 and is energized from a source 17 of alternating current. A source 18 of direct current operating voltage and a ballasting resistor 19 and isolating diode 20 are serially connected with igniter circuit 16 across output terminals 1 I, 12. The plasma power source 22 is coupled across the serially connected ballasting resistor 19 and insulating diode 20.

Referring now to FIG. 2, a series-ballasting resistor provides a linear voltage/current characteristic as shown in FIG. 2A. A high-reactance transformer employed for ballasting provides a nonlinear, exponential voltage/current characteristic as shown in FIG. 25. Employment of a series-ballasting resistor in conjunction with a high-reactance transformer provides a more linear, but still exponential voltage/current characteristic as shown in FIG. 2C. The negative, exponential, voltage/current characteristic of a typical arc discharge device is shown in FIG. 3, and it will be seen that the voltage/current characteristics of the resistive and high-reactance devices, as shown in FIGS. 2A, B and C, do not at all correspond to the lamp characteristic, and it will be observed that each of these ballasting techniques involves excessive energy loss.

Referring now to FIG. 4, a transformer 23 is provided having its primary winding 24 energized, preferably at 400 cycles, by inverter 25 coupled across low voltage direct current power source 18, which may be a 28 volt source.

Secondary winding 26 of transformer 23 has a center tap 27, and a plurality of progressively higher voltage taps 28, 29 and 30 respectively on opposite sides thereof. In the illustrated embodiment, three progressively higher alternating current voltages are provided by taps 28, 29 and 30. Center tap 27 is coupled to the positive terminal 32 of direct current source I8 by lead 33.

Three plasma voltage supply circuits 34, 35 and 36 are provided respectively having series ballast resistors 37, 38 and 39 therein. Plasma voltage supply circuits 34, 35 and 36 are respectively connected to taps 28, 29 and 30 of secondary winding 26 by diodes 40, 42 and 43 which thus provide fullwave rectification of the three progressively higher alternating current voltages respectively applied to the plasma voltage Section 53 of secondary winding 26 has its side 54 coupled to lead 44 by momentary start switch 55 and its other side 56 coupled thereto by capacitor 57. Spark gap 58 couples side 56 to tap 59 on ignition transformer winding 45.

A relay 60 is provided having a voltage-responsive operating coil 62 coupled across ballast resistor 19 and having normally closed contacts 63 coupled in series with primary winding 24 of transformer 23. Thus, when the current flow through resistor 19 and thus the voltage drop thereacross reaches a predetermined value, operating coil 62 will actuate contacts 63 to open the energizing circuit of primary winding 24. In order to start lamp 15, switch 55 is momentarily closed thereby coupling capacitor 57, section 64 of ignition transformer winding 45, and spark gap 58 across transformer secondary winding section 53, the capacitor 57 and spark gap 58 thereby in known fashion generating a high-voltage pulse in section 64 of transformer winding 45 to apply the requisite high-voltage starting pulse across the electrodes of lamp 15. While a manually actuated starting switch 55 is shown it will be understood that a conventional automatic starting switch may be employed.

Upon initial ionization of lamp l and opening of starting switch 55, rectified plasma current flows in diodes 43 and ballasting resistor 39 and plasma voltage is supplied by circuit 36, the voltage drop across resistor 39 initially being such that the potential in lead 44 is sufficiently high to back-bias diodes 40 and 42 so that current does not flow in supply circuits 34 and 35. As the plasma arc current increases, the voltage drop across ballast resistor 39 increases thereby lowering the potential of lead 44 until the back-bias on the diodes 42 is removed and those diodes then begin to conduct additionally supplying current through supply circuit 35. As the plasma arc current still further increases, the potential of lead 44 will finally decrease to the point where diodes 40 will begin to conduct thereby also supplying current through supply circuit 34.

It will thus be seen that a plurality of progressively lower direct current voltages are sequentially connected to the lamp so that, as the lamp is ionized, the highest voltage initially provides a small amount of current and as this voltage becomes loaded down, the next lower voltage tap will begin to provide current. Thus, as each tap of successively lower voltage provides current and that voltage is loaded down, the current through the lamp will be increasing until the lowest voltage tap is providing low voltage at a reasonably high current, such current being sufficiently high to insure ignition, even at low temperatures. it will be observed that diodes 40, 42, 43, resistors 37, 38 and 39, and lead 44 function as an OR circuit.

It will be observed that the plasma voltages are pedestaled upon the operating voltage. During the buildup of plasma arc current, the potential of lead 44 back-biases diode 20 so that direct current operating voltage source 18 is not directly coupled to the lamp circuit. As the lamp current approaches its operating value, the potential of lead 44 reduces to the point at which the back-bias on diode 20 is removed thus permitting source 18 to supply lamp current through ballast resistor 19 and diode 20. At this point, operating coil 62 of relay 60 actuates contact 63 to deenergize primary winding 24 of transformer 23 thus disabling the plasma power supply 22, the lamp thereafter being operated solely from direct current source l8 through ballast resistor 19 and diode 20.

Further, the values of ballast resistors 37, 38 and 39 are proportioned respectively to provide voltage/current characteristics 65, 66 and 67 as shown in FIG. 5, the resultant composite, stepped characteristic generally approximating the lamp characteristic, as shown in FIG. 3.

In a system for starting and operating a Hanovia 49lC-39 xenon lamp having a starting voltage of 20-30 kv., a minimum plasma voltage of 70 volts, and an operating voltage of 20-25 volts, the following voltages and component values may be employed in the circuit shown in FIG. 4:

open circuit voltage tap 28- lO volts open circuit voltage tap 29-35 volts open circuit voltage tap 30-90 volts resistor 370.5 ohm resistor 382.5 ohms resistor 39- 10 ohms resistor l9-0.06 ohm resistor 46- 10,000 ohms capacitor 47-200 mfd.

It will be readily understood that a greater number of transformer secondary taps and thus plasma supply voltages may be desired in certain circumstances, or in the alternative that two rather than three taps and plasma supply voltages may be sufficient.

Referring now to FIG. 6 in which like elements are indicated by like reference numerals, and in which only two transformer secondary winding taps 29, 30 and plasma voltage supply circuits 35, 36 are employed, it will be observed that ballast resistor 38 has been eliminated from the plasma voltage supply circuit 35, and replaced by ballast resistor 68 serially connected in the common lead 33. In this embodiment, again the initial plasma current flow through ballast resistors 68 and 39 is insufficient to reduce the potential lead 44 sufficiently to remove the back-bias on diode 42. However, the increased current flow flowing through both resistors results in unloading the highest voltage supply circuit 36 and removing the back-bias on diode 42.

It will now be seen that in accordance with the invention, as the lamp rapidly progresses from a very low current conduction to a high-current, low-voltage conduction, the combined voltage provided by the plasma power supply is progressively loaded and the current limited until the lowest voltage supply circuit is delivering a relatively high current at a low-voltage level.

While there have been described above the principles of this invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention.

What is claimed is:

l. A starting and operating system for an arc discharge device comprising an output circuit connected to said device, first means coupled to said output circuit for supplying a relatively high voltage for starting said device, second means coupled to said output circuit for supplying a first relatively low direct currentvoltage for operating said device and including first means for limiting the operating current flow in said device; and a direct current plasma power supply coupled to said output circuit and including a plurality of plasma voltage supply circuits for supplying a corresponding plurality of direct current voltages respectively progressively higher than said first voltage, each of said supply circuits including bias means for limiting the current flow therein said bias means being coupled to said output circuit and sequentially applying progressively lower ones of said plurality of voltages thereto in response to increasing current flow in said device following starting thereof.

2. The system of claim 1 wherein said device has a negative exponential voltage/current characteristic, said bias means being respectively proportioned to apply said progressively lower voltages to approximate said characteristic.

3. The system of claim 1 wherein each said bias means includes a current-limiting resistor and diode respectively coupled in series with each supply circuit.

4. The system of claim 1 wherein said bias means and supply circuits comprise an OR circuit.

5. The system of claim 3 wherein said plasma power supply includes means for providing a corresponding plurality of progressively higher alternating current voltages and having said supply circuits coupled thereto, said diode in each of said supply circuits providing a rectified alternating current voltage.

6. The system of claim 5 wherein said alternating current voltages providing means comprises a transformer secondary winding having a common point, each of said supply circuits having one side connected to different voltage points on said winding with respect to said common point, the other sides of said supply circuits being connected together and to said common point, each supply circuit including said diode and resistor serially connected in said one side, said common sides of said supply circuits being connected to one side of said output circuit and the one side of each of said supply circuits being connected through said winding to the other side of said output circuit, said resistors being proportioned so that only the diode connected in the highest voltage supply circuit is initially conductive upon starting of said device with the remaining diodes being initially back-biased out of conduction and being sequentially biased into conduction in response to the increase in current flow through said device.

7. The system of claim 6 including pedestal means for adding said plasma voltage to said first voltage.

8. The system of claim 6 wherein said low-voltage supplying means includes a source of direct current voltage having one side connected to said common side of said supply circuits and the other side thereof connected to said one side of said output circuit, said first current-limiting means comprising a resistor series-connected with an isolating diode between said one side of said source and said other side of said output circuit.

9. The system of claim 8 further comprising means responsive to a predetermined current flow in said last-named resistor for disabling said plasma power supply.

10. The system of claim 8 further comprising a primary winding coupled to said secondary winding, and an inverter circuit coupling said source to said primary winding.

1 l. The system of claim 6 wherein said first supplying means includes a second transformer secondary winding connected in series with said output circuit, and further comprising means coupling a pan of said first-named transformer winding to said second transformer winding for inducing said starting voltage therein.

12. The system of claim 5 wherein said alternating current voltages providing means comprises a transformer secondary winding, each of said supply circuits having first and second sides, the second sides being connected in common and to a midpoint on said winding, each said supply circuit including a pair of diodes respectively connecting the first sides of said supply circuits to corresponding points on said winding on opposite sides of said midpoint to provide full-wave rectification of the alternating current voltages respectively supplied to said supply circuits, each of said bias means comprising a current-limiting resistor connected in series with each said pair of diodes in one side of a respective supply circuit, said common sides of said supply circuits being connected to one side of said output circuit and the first sides of said supply circuits being connected through said winding to the other side of said output circuit, said resistors being proportioned so that only the pair of diodes connected to the highest voltage points on said winding are initially conductive upon starting of said device with the remaining pairs of diodes being initially back-biased out of conduction and being sequentially biased into conduction in response to the increase in current flow through said device.

13.,The system of claim 12 wherein one of said resistors is connected in series in said common side, the remaining resistors being connected in series in the first sides of respective supply circuits other than the circuit coupled to the lowest voltage point on said winding.

14. The system of claim 12 further comprising a filter circuit coupled across said supply circuits.

15. The system of claim 12 wherein said low-voltage supplying means includes a source of direct current voltage having one side connected to said common side of said supply circuits and the other side thereof connected to said one side of said output circuit, said first current-limiting means comprising a resistor and isolating diode connected in series between said one side of said source and said other side of said device, said first supplying means including a second transformer secondary winding connected in series with said output circuit and further comprising means coupling a part of said first-named transformer winding to said second transformer winding for inducing said starting voltatge therein, and means responsive to a predetermined current ow in said last-named resistor for disabling said plasma power supply.

16. The system of claim 12 wherein said device has a negative exponential voltage/current characteristic, said firstnamed resistors being respectively proportioned to apply said progressively lower voltages to said output circuit to approximate said characteristic. 

1. A starting and operating system for an arc discharge device comprising an output circuit connected to said device, first means coupled to said output circuit for supplying a relatively high voltage for starting said device, second means coupled to said output circuit for supplying a first relatively low direct current voltage for operating said device and including first means for limiting the operating current flow in said device; and a direct current plasma power supply coupled to said output circuit and including a plurality of plasma voltage supply circuits for supplying a corresponding plurality of direct current voltages respectively progressively higher than said first voltage, each of said supply circuits including bias means for limiting the current flow therein said bias means being coupled to said output circuit and sequentially applying progressively lower ones of said plurality of voltages thereto in response to increasing current flow in said device following starting thereof.
 2. The system of claim 1 wherein said device has a negative exponential voltage/current characteristic, said bias means being respectively proportioned to apply said progressively lower voltages to approximate said characteristic.
 3. The system of claim 1 wherein each said bias means includes a current-limiting resistor and diode respectively coupled in series with each supply circuit.
 4. The system of claim 1 wherein said bias means and supply circuits comprise an OR circuit.
 5. The system of claim 3 wherein said plasma power supply includes means for providing a corresponding plurality of progressively higher alternating current voltages and having said supply circuits coupled thereto, said diode in each of said supply circuits providing a rectified alternating current voltage.
 6. The system of claim 5 wherein said alternating current voltages providing means comprises a transformer secondary winding having a common point, each of said supply circuits having one side connected to different voltage points on said winding with respect to said common point, the other sides of said supply circuits being connected together and to said common point, each supply circuit including said diode and resistor serially connected in said one side, said common sides of said supply circuits being connected to one side of said output circuit and the one side of each of said supply circuits being connected through said winding to the other side of said output circuit, said resistors being proportioned so that only the diode connected in the highest voltage supply circuit is initially conductive upon starting of said device with the remaining diodes being initially back-biased out of conduction and being sequentially biased into conduction in response to the increase in current flow through said device.
 7. The system of claim 6 including pedestal means for adding said plasma voltage to said first voltage.
 8. The system of claim 6 wherein said low-voltage supplying means includes a source of direct current voltage having one side connected to said common side of said supply circuits and the other side thereof connected to said one side of said output circuit, said first current-limiting means comprising a resistor series-connected with an isolating diode between said one side of said source and said other side of said output circuit.
 9. The system of claim 8 further comprising means responsive to a predetermined current flow in said last-named resistor for disabling said plasma power supply.
 10. The system of claim 8 further comprising a primary winding coupled to said secondary winding, and an inverter circuit coupling said source to said primary winding.
 11. The system of claim 6 wherein said first supplying means includes a Second transformer secondary winding connected in series with said output circuit, and further comprising means coupling a part of said first-named transformer winding to said second transformer winding for inducing said starting voltage therein.
 12. The system of claim 5 wherein said alternating current voltages providing means comprises a transformer secondary winding, each of said supply circuits having first and second sides, the second sides being connected in common and to a midpoint on said winding, each said supply circuit including a pair of diodes respectively connecting the first sides of said supply circuits to corresponding points on said winding on opposite sides of said midpoint to provide full-wave rectification of the alternating current voltages respectively supplied to said supply circuits, each of said bias means comprising a current-limiting resistor connected in series with each said pair of diodes in one side of a respective supply circuit, said common sides of said supply circuits being connected to one side of said output circuit and the first sides of said supply circuits being connected through said winding to the other side of said output circuit, said resistors being proportioned so that only the pair of diodes connected to the highest voltage points on said winding are initially conductive upon starting of said device with the remaining pairs of diodes being initially back-biased out of conduction and being sequentially biased into conduction in response to the increase in current flow through said device.
 13. The system of claim 12 wherein one of said resistors is connected in series in said common side, the remaining resistors being connected in series in the first sides of respective supply circuits other than the circuit coupled to the lowest voltage point on said winding.
 14. The system of claim 12 further comprising a filter circuit coupled across said supply circuits.
 15. The system of claim 12 wherein said low-voltage supplying means includes a source of direct current voltage having one side connected to said common side of said supply circuits and the other side thereof connected to said one side of said output circuit, said first current-limiting means comprising a resistor and isolating diode connected in series between said one side of said source and said other side of said device, said first supplying means including a second transformer secondary winding connected in series with said output circuit and further comprising means coupling a part of said first-named transformer winding to said second transformer winding for inducing said starting voltage therein, and means responsive to a predetermined current flow in said last-named resistor for disabling said plasma power supply.
 16. The system of claim 12 wherein said device has a negative exponential voltage/current characteristic, said first-named resistors being respectively proportioned to apply said progressively lower voltages to said output circuit to approximate said characteristic. 